Remove \brief commands from doxygen comments.

We've been running doxygen with the autobrief option for a couple of
years now. This makes the \brief markers into our comments
redundant. Since they are a visual distraction and we don't want to
encourage more \brief markers in new code either, this patch removes
them all.

Patch produced by

      for i in $(git grep -l '\\brief'); do perl -pi -e 's/\\brief //g' $i & done

lib/Frontend/CompilerInvocation.cpp

@@ -114,7 +114,7 @@ static void diagnoseSwiftVersion(Optional<version::Version> &vers, Arg *verArg,
   }
 }
 
-/// \brief Create a new Regex instance out of the string value in \p RpassArg.
+/// Create a new Regex instance out of the string value in \p RpassArg.
 /// It returns a pointer to the newly generated Regex instance.
 static std::shared_ptr<llvm::Regex>
 generateOptimizationRemarkRegex(DiagnosticEngine &Diags, ArgList &Args,